Better form validation

Besides https://github.com/jtnord/cursor-origin-branch-source-plugin/blob/a8b5e29c70e540c1b07486b5974e52a6ada6415f/src/main/java/io/jenkins/plugins/cursor_origin_branch_source/CursorOriginAppCredentials.java#L264 there ought to be validation on owner/repo fields given the credentials id, to make sure the specified objects actually exist and are accessible to that app.

Start in src/main/java/io/jenkins/plugins/cursor_origin_branch_source/CursorOriginAppCredentials.java around line 264, where the current form validation is referenced. Trace how the credentials id is used, then verify that the supplied owner and repository exist and are accessible to that app; the work is done when invalid or inaccessible objects are rejected during validation.

Written by the indexing model from the issue text.